Upon differentiation of embryonal carcinoma cells induced by retinoids, the "apparent" membrane microviscosity increases dramatically. Only biologically active retinoids induced differentiation and caused an enhancement in microviscosity. Several embryonal carcinoma cell lines have a relatively lower "apparent" microviscosity than their differentiated derivatives suggesting that, in general, differentiation is accompanied by an increase in microviscosity. At higher concentrations retinoids cause a reduction in "apparent" membrane microviscosity of various cells, whether the analogue is biologically active or not. The cell surface appears to be a target for the action of retinoids and tumor promoting agents. These groups of compounds were found to exert opposing actions on the availability of epidermal growth factor receptors and on the biosynthesis and secretion of procollagen in a variety of cell lines. Generally, retinoids had an enhancing effect on these parameters, whereas tumor pormoters had an inhibitory effect.